Blood agar is actually a couple of related media, all of which are rich formulations containing peptones, yeast extracts, liver or heart extracts (depending on the medium), and blood. The blood is usually sheep's red blood cells (RBC), though horses and other species may be used. Blood agar is used to grow fastidious organisms (species that do not grow easily) requiring a rich media providing many nutrients and growth factors that are largely supplied by blood. It is also a differential media in allowing the detection of hemolysis (destroying the RBC) by cytolytic toxins secreted by some bacteria, such as certain strains of Bacillus, Streptococcus, Enterococcus, Staphylococcus, and Aerococcus.